CAS Key Laboratory of Engineering Plastics

A New Progress in Confined Crystallization

 Dujin Wang's Group Acquired a New Progress in Confined Crystallization

and Phase Transition of Polymer and Its Model Compound

 

Crystallization is an important physicochemical feature for polymeric materials, since the crystallinity and superstructure morphology greatly influence thermal and mechanical properties that in turn determine practical applications. However, it is difficult to propose a quantitative formula for describing the transition dynamics in the crystalline polymeric systems, due to the well-known reasons of polydispersity of molecular weight distribution, side chain architecture, and the existence of multiple hierarchical structures as well as different domain sizes in the polymer system. As well-defined model systems, normal alkanes (n-alkanes) and comb-like polymers offer an opportunity to approach the complicated crystallization process of polymer materials, surfactants, and lipids.

 

Since 2006, researchers in the laboratory of Dujin Wang’s group have developed an ingenious technique for synthesizing nearly monodispersed n-alkane containing microcapsules with controllable size and surface porous morphology by in-situ polymerization method, using the melamine−formaldehyde resin as shell materials and nonionic surfactants as emulsifiers. These investigations revealed a new direct evidence for the existence of surface freezing in microencapsulated n-alkanes. By elucidating the differences among chain packing and nucleation kinetics between bulk alkane solid solutions and their microencapsulated counterparts, they also discovered a mechanism responsible for the formation of a new metastable bulk phase. In addition, they found that the confinement suppresses lamellar ordering and longitudinal diffusion, which play an important role in stabilizing the binary n-alkane solid solution in microcapsules. Based on these studies, the confined crystallization behavior of a low molecular weight monodisperse polyethylene oxide (PEO) in anodic alumina oxide (AAO) templates was investigated. When AAO diameter decrease to a value smaller than the contour length of PEO, unique orientation with chain axis aligned parallel to the pore axis was observed for the first time.

 

 

Comb-like polymers with flexible side chains chemically pended onto a different rigid polymeric backbone afford some unusual properties due to their hierarchical structures, such as nanoscale confined crystallization, phase transition and conformational variations, length scale effects, etc. By designing the rigidity of polymeric backbone and the side-chain length of comb-like polymer, they found the chain crystallization, relaxation and arrangements, as well as topological architecture of molecular chains exhibited a hierarchical transition process against the bulk ones. In their studies, an interesting structure-evolution process, and phase transformation between orthorhombic, monoclinic, hexagonal phase, and amorphous state was demonstrated, indicating that size-dependent alkyl domains have a determination role for these complicated structures. The firstly formed conformational disordered structure before crystallization provides a deep understanding and direct support for the viewpoint of early stage of polymer crystallization, especially for the formation mechanism of metastable structure during polymer crystallization.

 

 

These groundbreaking results were reported in two review articles (Acc. Chem. Res. 2013, DOI: 10.1021/ar400116c; Chem. Soc. Rev. 2013, 42, 2075-2099) this year.

CAS Key Laboratory of Engineering Plastics